The invention relates to a diode structure, especially for use for thin-film solar cells.
Thin-film solar cells on the basis of polycrystalline semiconductors offer good chances of significantly reducing the costs for producing rugged and highly efficient solar modules. Of all thin-film solar cells, those on the basis of chalcopyrite semiconductors feature hitherto highest efficiency and count as an interesting candidate for future low-cost solar current or photovoltaic systems. Chalcopyrite compounds in this respect include compounds of the group Cu(InGa)(Sse)2, and more particularly copper indium diselenide (CuInSe2).
A typical layer structure of a chalcopyrite cell is shown in FIG. 4a. On the p-conducting chalcopyrite semiconductor, a CdS layer forms a heterojunction whose electrical field permits charge carrier separation. The front contact on top thereof is formed by a ZnO layer and the back contact is formed by a layer of molybdenum on an insulating substrate, such as e.g. glass.
Illustrated in FIG. 4b is the corresponding band diagram relative to the structure as shown in FIG. 4a, it being evident that the ZnO window layer comprises a substantially larger band gap. This prevents photogenerated charge carriers from being absorbed directly at the surface of the solar cell, and due to the high defect density, immediately recombining there. The heterostructure thus results in substantially greater penetration depths and higher photocurrent yield. This requires, however, that the p-conducting absorber and the n-conducting window layer are well adapted both structurally and electronically.
Window layers as known and tested in production are based on doped metal oxides such as e.g. ZnO, SnO2 or InSnO2 (ITO) generally termed transparent conductive oxides (TCO). Known TCO layers, however, are difficult to adapt with regard to lattice constant or electron affinity to chalcopyrite semiconductors. This is why directly combining these window layers with chalcopyrite absorbers has hitherto failed to yield high and reproducible solar cell efficiencies.
To improve adapting TCO and absorber, thin, i.e. only approx. 50 nm thick, buffer layers are usually inserted between absorber layer and window layer. Best electronic quality and high efficiency is achieved by a diode configuration consisting of a chalcopyrite absorber, CdS buffer layer, and ZnO front electrode, it being this solar cell structure that, by far, achieves the highest efficiency of all thin-film solar cells (up to 18.8%). In addition, this solar cell structure features maximum process tolerance with regard to layer thickness and thus the high yield in production.
However, due to the CdS buffer layer, chalcopyrite semiconductors hitherto most successful contain heavy metals which complicate production and disposal.
It is thus the objective of the invention to provide a diode structure for thin-film solar cells achieving a configuration of the thin-film solar cell as simple as possible for high efficiency in using materials offering good environmental compatibility.
According to one embodiment of the invention, a diode structure for thin-film solar cells comprises a p-conducting layer comprising a chalcopyrite compound. The diode structure further comprises a n-conducting layer having a first band gap, the n-conducting layer further comprising a compound, the compound containing titanium and oxygen. The n-conducting layer adjoins the p-conducting layer The diode structure also comprises a n-conducting amplifying layer having a second band gap. Further, according to this embodiment, a side of the n-conducting layer facing away from the p-conducting layer adjoins the n-conducting amplifying layer, and the second band gap is larger than the first band gap.
According to one aspect of some embodiments of the invention, the chalcopyrite compound is a I-III-VI2 semiconductor from the group Cu(InGa)(SSe)2.
According to another aspect of some embodiments of the invention, the chalcopyrite compound comprises CuInSe2 (CIS).
According to another aspect of some embodiments of the invention, the compound containing titanium and oxygen is selected from a group TiOx where x is in the range from greater than 1.5 to less than 2.0.
According to another aspect of some embodiments of the invention, the compound containing titanium and oxygen is selected based on the chalcopyrite compound, so as to achieve a best possible adaptation in a conduction band.
According to another aspect of some embodiments of the invention, the n-conducting amplifying layer comprises an oxide that is transparent and conductive.
According to another aspect of some embodiments of the invention, the oxide is a doped metal oxide.
According to another aspect of some embodiments of the invention, the oxide comprises any one of ZnO, SnO2 and InSnO2.
According to another embodiment of the invention, a thin-film solar cell comprises a diode structure. The thin-film solar cell comprises a p-conducting layer comprising a chalcopyrite compound. The thin-film solar cell further comprises a n-conducting layer having a first band gap, the n-conducting layer further comprising a compound, the compound containing titanium and oxygen. The n-conducting layer adjoining the p-conducting layer The thin-film solar cell also comprises a n-conducting amplifying layer having a second band gap. Further, according to this embodiment, a side of the n-conducting layer facing away from the p-conducting layer adjoins the n-conducting amplifying layer, and the second band gap is larger than the first band gap.
According to one aspect of some embodiments of the invention, a layer thickness of the n-conducting layer is selected based on a sheet resistivity of the n-conducting layer.
According to another aspect of some embodiments of the invention, a side of the p-conducting layer facing away from the n-conducting layer adjoins a bus contact.
According to another aspect of some embodiments of the invention, the n-conducting amplifying layer adjoins a substrate, the n-conducting amplifying layer facing away from a light incident side of the substrate.
According to another aspect of some embodiments of the invention, the bus contact comprises molybdenum.
According to another aspect of some embodiments of the invention, the bus contact is configured as a full surface area back electrode.
According to another aspect of some embodiments of the invention, the bus contact adjoins a light incident side of a substrate.
According to another aspect of some embodiments of the invention, the substrate comprises glass.
Other advantages, novel features, and objects of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, which are schematic and which are not intended to be drawn to scale. In the figures, each identical, or substantially similar component that is illustrated in various figures is represented by a single numeral or notation. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention.